The present invention relates generally to the packaging of small-scale integrated circuits. More particularly, the invention relates to leadless leadframe package designs.
A leadless leadframe package (LLP) is a relatively new integrated circuit package design that contemplates the use of a metal (typically copper) substrate in the formation of a chip scale package (CSP). As illustrated in FIGS. 1A and 1B, in typical leadless leadframe packages, a copper leadframe strip or panel 10 is patterned to define a plurality of arrays or matrixes 11 of semiconductor device areas 12. Each device area 12 includes a die attach pad 13 and a plurality of contacts 15 disposed about their associated die attach pad 13. Very fine tie bars 16 are used to support the die attach pads 13 and contacts 15.
FIG. 2 illustrates a cross-sectional view of a typical resulting leadless leadframe package 14. The die attach pad 13 supports a die 17 which is electrically connected to its associated contacts 15 by bonding wires 18. A molded cap 20 encapsulates the die 17 and bonding wires 18, and fills the gaps between the die attach pad 13 and the contacts 15 thereby serving to hold the contacts 15 in place. It should be appreciated that during singulation of the individual packages 14, the tie bars 16 are cut and therefore the only materials holding the contacts 15 in place is the molding material. The resulting packaged chip can then be surface mounted on a printed circuit board or other substrate using conventional techniques.
FIG. 3A illustrates a bottom plan, perspective view of an improved leadless leadframe package 14xe2x80x2, while FIG. 3B illustrates a fragmentary and enlarged view of one of the contacts 15 and the molding material immediately surrounding the contact 15. In this improved design, an edge portion of each contact 15 has been removed to define a stem 26, which extends from an interior surface 25 of the contact 15 toward a respective peripheral surface 24. The molding material surrounds the stems 26 while leaving a peripheral surface 27 of the stems 26 exposed on the peripheral surfaces 24 of the package 14xe2x80x2. The portion of the molded cap 20 that covers the bottom surface 28 of the stem 26 is indicated by the phantom lines 29, and is referred to as the bar portion 30. These bar portions 30, integrally formed into the molded cap 20, function to lock the contact 15 into the molded cap 20 so that they are not pulled out by the singulating saw blade during the singulation process. Furthermore, the thinned peripheral edges of the contacts 15 reduce stress on the singulating saw blade since it is only required to cut through a smaller amount of metal during the singulation process. For further details, refer to U.S. patent application Ser. No. 09/470,826.
Although the contacts 15 of this package design 14xe2x80x2 provide the above-mentioned advantages, the bar portions 30 have been found to occasionally break-away from the molded cap 20 by the frictional forces introduced during singulation. This results in structurally damaged leadless leadframe packages. Consequently, it would be desirable to further improve leadless leadframe package designs to have a greater degree of structural integrity.
The present invention is directed towards a leadless leadframe package that has a greater degree of structural integrity such that portions of the molded cap do not break off during singulation processes. One aspect of the invention pertains to a leadless leadframe package that has a plurality of contacts, which have contact surfaces on the bottom surface of the package. At least some of the contacts have integrally formed stems that extend outward to the peripheral surface of the package. These stems have heights and widths less than the heights and widths of their corresponding contacts. An integrated circuit die is electrically coupled to at least some of the contacts. A molded cap encapsulates at least a portion of the die, the stems and the contacts. The molded cap leaves the contact surfaces of the contacts exposed on the bottom surface of the package, leaves a peripheral surface of the stems exposed on the peripheral surface of the package, and covers a bottom surface of each of the stems.
Another aspect of the invention pertains to a leadless leadframe panel assembly having a conductive substrate panel that has at least one array of device areas, each array of device areas having a plurality of tie bars and a plurality of contacts. The contacts have contact surfaces on the bottom surface of the leadless leadframe panel assembly. The contacts also have integrally formed stems that extend towards and connect to one of the tie bars. The stems have widths and heights that are less than the widths and heights of their corresponding contacts. A plurality of dice are mounted on the leadframe panel and each die is electrically connected to contacts within an associated device area A molded cap encapsulates at least a portion of each die, the tie bars, the stems and the contacts while leaving the contact surfaces of the contacts exposed and covering a bottom surface of each of the stems.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures that illustrate by way of example the principles of the invention.